Gas plasmas formed by glow discharge techniques in evacuated vessels have been used to modify the surfaces of various materials, including plastics, metals and ceramics. In particular, gas plasmas have been useful for depositing coatings such as organic polymers onto surfaces of workpieces such as films, fibers, prosthetic devices and so forth. A workpiece is positioned in a reaction vessel. The vessel is evacuated. A reactive gas or a mixture containing one or more reactive gases is then introduced at a pressure of 0.01 to 10 torr and excited to a plasma state, whereupon it modifies the surface of the workpiece. When the reactive gas or gas mixture contains a monomer or mixture of monomers that can form polymerizates from contact of the gas plasma with a surface, the surface will be coated with polymerizate deposits. Because of the nature of gas plasmas, coatings are also deposited on all other incidental surfaces in the reaction vessel, including vessel walls, any electrodes in the vessel, workpiece mounts, and the like. Electrodes placed within the reaction vessel are particularly prone to build-up of deposits. Furthermore, flaking of polymerizate deposits from metallic surfaces is commonly encountered because of poor adhesion of such deposits to metals, and this problem is pronounced with metallic electrodes.
As these deposits often build up quickly, especially in any continuous treatment process such as would be sought in the commercial use of this treatment technique, various difficulties have resulted. These include formation of loose flakes and debris that spall from thick coatings, opacification of window ports, and carryover of unwanted chemical elements to subsequent plasma coating operations. For example, treatment of a workpiece with a fluorinated monomer gas plasma, followed by treatment of a second workpiece with a nonfluorinated monomer gas plasma can result in contamination of the second workpiece with fluorine-containing species ablated from wall deposits. Wall deposits also undergo shrinkage due to continued exposure to gas plasmas. Because these deposits are exceptionally adherent to wall surfaces, shrinkage forces are transmitted to the walls. These forces have been known to cause crack formation in the walls with attendant vacuum loss, especially when the walls are made of transparent materials such as glass or synthetic quartz.
Japanese patent no. 63-246814 describes an apparatus wherein a film is guided around the inner wall of a cylindrical reaction vessel during gas plasma coating operations. The film feed roller and take-up rollers are mounted within the reaction vessel, as well as film guides along the film path. This design does not solve the problems of debris formation and cross-contamination, in that deposits would still build up on the electrode surfaces, the exposed guides and the rollers. An apparatus is needed that inherently eliminates the build up of unwanted plasma deposits at any point in the gas plasma treatment zone. Such an apparatus is needed in order to achieve the promise of continuous processing of articles by this technique.